1. Field of the Invention
The present invention relates to a composite particle which converts a low-energy light to a high-energy light, a composite particle dispersion prepared with the composite particle, and a photovoltaic device prepared with the composite particle dispersion.
2. Description of the Related Art
A solar cell has advantages that the amount of carbon dioxide emitted per power generation amount is small and it is not necessary to use fuel for power generation. Therefore, the solar cell has been hoped as an energy source to inhibit global warming. Currently, among solar cells in practical use, a mono-junction solar cell having a pair of p-n junction and using a single-crystal silicon or a polycrystal silicon has become a mainstream. In order to improve performance of the solar cells, research and development of various types of solar cells have been conducted.
An upconversion-type solar cell (hereinafter sometimes referred to as “UC type solar cell”) is one of the solar cells in which the performance can be improved. The upconversion-type solar cell includes a wavelength conversion member which converts long wavelength light having a low energy and resulting in light transmission loss, to short wavelength light having an energy which can be used with a solar cell material (upconversion). The wavelength conversion member includes a wavelength conversion substance which converts the long wavelength light to the short wavelength light. As the wavelength conversion substance, a fluorescent material including a rare earth ion (hereinafter the fluorescent materials are sometimes referred to as “rare earth phosphor”) and the like is known.
The upconversion easily occurs in the rare earth phosphor. However, since the band of light which can be absorbed by the rare earth phosphor is narrow, most parts of the band of the solar light cannot be used for the rare earth phosphor. In order to compensate this defect, it can be considered to use a semiconductor quantum dot (hereinafter simply referred to as “quantum dot”) together with the rare earth phosphor. The quantum dot can absorb light having a wide band of solar light spectrum, whereby it is possible to convert the light to light which can be absorbed by the rare earth phosphor with a relatively high efficiency.
As a technique related to such a solar cell, for example Non-Patent Document 1 discloses a technique of using a layer including a rare earth ion and PbS quantum dot, as a layer upconverting long wavelength light.